Detector circuit



May 19, 1936:

J. K. JOHNSON DETECTOR CIRCUIT,

Fil ed March 31, 1932 2 Sheets-Sheet 1 INVENTOR JOHN K. JOHNSON May 19, 1936. J. K. Jol -lNso N 2,041,291

DETECTOR CIRCUIT Filed March 31, 1932 2 Sheets-Sheet 2 INVENTQR JOHN K.JOHNSON BY 79, MM W MATTORNEYS Patented May 19, 1936 UNITED. STATES PATENT oFFics DETECTOR John Kelly Johnson, Bye, N. 1n, assignor to Hazeltine Corporation Application March 31, 1932, Serial No. 602,219 20 Claims. (01. 250-20) particularly to means for automatically regulat ing the output level of 'a'. radio receiver irrespective of reasonable changes in the magnitude of the desired carrier.

In automatic amplification control circuits heretofore proposed, it has been found extremely diflicult to secure adequate control when the tubes Y in the receiver are few in number and particularly when the strength of the incoming signal is great. The characteristic operation of the instrument under these conditions is to cause such serious distortion as often to render the signals unintelligible. v

It has been found that these distortion eifects are usually caused by detector overload, which is specifically due to both'grid and plate circuit rectification. These effects tend to operate' capable of producing high direct curren'tpoten tials for the purpose of automatically controlling 35 the amplifying characteristics of other tubes in the receiver. I n

These and further objects of the present invention will become apparent from the followingspecification taken in conjunction with the accompanying drawings. n

In accomplishing the objects of this invention, a low impedance variable mu triode in which the geometrical relationship between the several parts is such as to produce an exponential or substantially exponential grid voltage/plate current characteristic' at normal operating voltages, may be employed. Methods of accomplishing this result may include the following, among others: f 50 A. Varying'the pitch of the turns on the control rid. V

B. Forming the control grid in such manner that it exerts a changing influence on the electron stream over the length of the cathode. f C. Altering the spacing between turns in a unicause it to function as a triode.

fcrmly spaced control grid, such as by bending them out of position, etc.

In practice, it has been found convenient to employ a variable mu high impedance tetrode with the screen and plate connected together to 5 Such a tube may be caused to have a variable mu characteristic by any of the methods enumerated in items A, B, and/or C.

Whenever the term variable mu tube is used 10 in this specification and in the claims, there is meant a tube which, by any method such as those outlined above, operates in such a manner that the plate curren't varies substantially exponentially with grid voltage; that is, a tube having :15

tion in amplification factor, usually of the order of a few per cent, which is toosmall to. serve any useful purpose.

In operation, the detector is arranged as a grid leak and condenser detector, or, as it is more commonly expressed, as a grid rectifier.

Under conditions of operation in which there is either no incoming signal or the incoming signal applied to the input of the detector is quite weak, the current in the plate circuit remains at a steady maximum value, and at the particular operating potentials the voltage amplification or mu of the tube may, for example, be of the order of from ten to fifteen. If 1 the incoming signal voltage applied to the grid of the detector is then greatly increased, the grid will assume a high negative 40 potential, which will in turn cause a substantial reduction in the flow of current in the plate circuit and thus greatly reduce the mu or the voltage amplification of the tube. Within limits, therefore, the mud the detector tube and its audio amplifying properties may be made to vary depending upon the strength of the signals. This of itself tends to produce a partial or automatic regulation of the output intensity.

it is well understood that the ordinary grid circuit rectifiers may be made to function quite satisfactorily up to a critical voltagelevel, but that beyond this point they produce severe distortion. This is by virtue. of the fact that at high signal intensities a negative voltage is developed in the grid circuit which tends to so reduce the fiow of current in the plate circuit as to cause operation about the knee of the plate current curve. At this portion of the curve, plate rectification takes place, which is in opposition to the grid rectification, thus causing the distortion effects previously mentioned and also causing extreme insensitivity. This condition continues as the signal intensity continues to increase, until such time as the major portion of the rectification eiiect occurs in the plate circuit.

In the present invention, the undesirable effects of this double, opposing, rectifying action are reduced by the combination of geometrical arrangement of the component elements of the tube. with the proper circuit constants, so that the grid voltage/plate current characteristic has a gradual curve over its entire excursion, thus minimizing the usual plate circuit rectification.

The invention has particular application in tuned radio-frequency receivers and superheterodyne receivers, which may or may not include 'control receivers, having circuits designed to automatically control the output level irrespective of moderate changes in the magnitude of the carrier frequency, the application of the invention is particularly useful because it makes possible a high direct-current potential which can be impressed on the control circuits of other amplifying tubes to automatically regulate their amplifying properties.

By virtue of the fact that the effective sensitivity of one or more of the detectors can be made to vary as a function of the signal voltage applied thereto, it has been found possible to construct an automatic amplification control broadcast receiver with, for example, as few as two of the amplifying tubes controlled by the direct-current control voltage derived from the rectified carrier frequency component, while the additional control secured through the drop in sensitivity of the one or more detectors has made it possible to secure an overall performance which meets all practical requirements.

Referring now to the drawings:

Fig. 1, is a circuit diagram of a superheterodyne radio receiver showing the detector tubes conand nected to operate in accordance with the present invention;

Figs. la. and 1b are diagrams of tube arrangements adapted to be used for the detector tubes in Fig. 1;

Fig. 2 is a diagram showing the operation of the detector constituting the present invention,

Fig. 3 is an overload characteristic curve of such a detector.

, Referring now to Fig. 1, there is shown a superheterodyne radio receiver, arranged for automatic volume control. The received radio-frequency signals are amplified by the radio frequency amplifier tube l0 and its associated circuits. The amplified signals are detected in the When used as the first detector II,- the heterodyne or oscillation current being supplied by the circuits of oscillator tube I4. The intermediate radio-frequency signals are amplified by the circuits of the intermediate frequency amplifier tube I6 and are supplied to the tuned input circuit 20 of the second detector l8. The radio frequency amplifier, oscillator, first detector, and intermediate frequency amplifier circuits, are, except as noted hereinafter, of the usual, well-known type and need not be further described. The detector tubes l2 and I8 may both have a variable mu. They may be constructed by any one of the methods set forth above. Fig. la shows such a tube in which the grid I! is constructed having its turns variably spaced. Alternatively, a variable mu type tetrode, such, for instance, as a type 235 tube, having its screen-grid electrode and its plate or anode connected together, may be used as the second detector. Such a tube is shown in Fig. 11).

An input circuit, including the tuned circuit 20, is connected between the input or grid electrode and the cathode of the tube l8. Included between the tuned circuit 20 and the cathode of the detector tube I8 is grid-leak resistor 2i shunted by grid condenser 22. The grid-leak resistor 2| and condenser 22 are so proportioned that the tube l8 will operate to give grid detection.

The rectified grid bias voltage existing across the resistor 2| is fed through the resistor 26 to regulate the input grid bias of the tube It and through the resistor 28 to regulate the input grid bias of the tube Ill and thus to control the amplification of the intermediate frequency amplifier and radio frequency amplifier, respectively. The normal or initial biases of the tubes l0 and I6, as provided by any appropriate biasing source, such as the biasing batteries II and I5 respectively, are adjusted to give maximum amplification, as is usual in automatic amplification control receivers, and the automatic control connection is so arranged that an increase in the voltage across the grid-leak II will increase the negative bias of the amplifier tubes and thus reduce their amplification as required.

The output circuit, which is connected to the output electrode or anode and cathode of the detector ll, includes the lower portion of the choke III, the resistor 32, and the high potential source 34. A high frequency by-pass condenser 38 is provided around the output of the vacuum tube ll, and an audio frequency by-pass condenser 36 is arranged to by-pass the audio frequency around the battery 34. The choke 30, which should preferably have an alternating current impedance at least as high as the minimum alternating current impedance of the tube, acts as an auto-transformer, and the audio frequency voltage produced is transmitted through the resistor 40 and the coupling condenser 42 to the level control resistor H, and through the biasing battery 48 to ground. A radio-frequency by-pass condenser 48 to ground is provided between the resistor 40 and the coupling condenser 42. The resistor 40 and the condenser ll are so proportioned that any residual radio frequencies will be substantially eliminated. The variable tap of the resistor 44 is connected to the grid of the amplifier tube 9 for impressing any desired portion of the voltage drop of said resistor upon the input of the audio frequency amplifier. The output of the amplifier may be connected through terminals 50 to a loud-speaker or the following audio frequency amplifiers, as required. The

Inexplaining the operation of the circuit of Fig. 1, attention is now invited to Fig. 2, in which the ordinates represent current and the abscissas represent grid voltage. The curve 52 is the plate current curve of a high mu type amplifier, and the curve 53 is a similar curve of a low mu type amplifier. The grid current characteristic curve is shown at 54. The voltages impressed upon the grid of a tube are represented below the origin 0, the curve 55 representing one cycle of voltage variation of a radio frequency current impressed upon theinput of a detector tube, for

instance. The curve 55' is the resulting grid current curve of the detector. This surge of current charges the grid condenser 22, for instance, of Fig. 1, and establishes a negative grid bias, which results in moving the axis of the incoming carrier frequency voltage impressed upon the grid from CA to O'A'. Further high frequency oscillations impressed upon the grid are then represented by the curve 55, which produces a rectified grid current represented by curve 56'. However, ascan be seen, if the detector tube is of the lliighf'mu type, thie impressd carrier frequency oscillation voltage represented by curve 56 results in a rectified plate current represented b curve 51. Therectifled plate current is 180 out of phase with the rectified grid current, or;

age of the tube used as a detector and the abscissas represent input voltage. The curve 80 shows the overload characteristic of a high mu tube, which indicates overload at a relatively small input voltage, as explained above.

On the other hand, if the detector tube is of the low mu type, having a'plate or anode characteristic as represented by curve 53, in Fig. 2, a considerable negative grid bias can be obtained before there is any plate rectification, and therefore there will be no overloading until a very heavy grid voltage is obtained. The overload characteristic curve of the low mu type tube is shown at GI in Fig. 3.

By using a variable mu detector tube, the characteristic of which is indicated by the dotted curve 58 of Fig. 2, a much better amplification factor is obtained for small negative grid biases representing a srnall input voltage, and yet the overload characteristic of such a tube is such that overloading will not be obtained except for extreme negative grid biases. The overload characteristic of such a tube is'shown by the dotted curve 82 ,of Fig. 3. From this curve it can beseen that the ratio between radio-frequency and audio-frequency voltage fluctuations varies continuously with the input radio-frequency voltage,

and at an approximately uniform rate, as the radio-frequency voltage increases to a large magnitude.

Thus, without sacrificing any substantial amplification at low input voltages, overloading is prevented except at very high input voltages, which permits a considerable bias to be built up across 'the grid-leak resistor 2|. Hence, by use of a variable mu'type tube for the detector l8, sufficient voltage drop may be obtained to give full volume control to the radio frequency and intermediate frequency amplifier tubes I0 and I5, re spectively, and the detector itself assists in the amplification control of the signals beingamplified. In addition, a considerable amplification of the detected signals is obtained in the detector l8. The use of a variable mu tube for the detector l2 assists the automatic amplification control action and prevents interference between plate and grid circuit rectification, with the consequent distortion, when receiving strong signals.

Although there is shown in the circuit diagram of Fig. 1 a detector circuit which is coupled to a following tube through an inductive coupling, in certain instances, such,'for example, as when the circuit is designed to be included in an alternating current socket power receiver, where the available voltage supply to the tubes may be made high, resistance coupling between the variable mu detector and the following tube is acceptable. In battery-operated or direct current receivers, where the available voltage supply is low, the coupling between the detector and following tubes includes a substantialhigh-frequency impedance connected between the anode and cathode of the detector, by which means the alternating current reactance is extremely high in comparison with the direct-current resistance. In this way, practically the entire plate voltage may be efiectively applied to the plate of the detector tube, thus producing the maximum voltage amplification and the minimum tube resistance for weak signals, while at the same time making it possible to secure a substantial drop in amplification and an increase in plate resistance of the tube when the signal voltage applied to the input of the detector is greatly increased.

What is claimed is:

1. An automatic amplification control radio receiver circuit, which comprises a detector circuit including a variable mu vacuum tube having cathode, grid, and anode, ,a tuned input circuit connected to said grid, a grid-leak resistor connected between said cathode and said tuned input circuit, and a condenser in shunt with said grid-leak resistor, a radio-frequency amplifier circuit the output of which is coupled to said tuned input circuit, connections from the input of said amplifier circuit to terminals of said grid-leak resistor, whereby the voltage across said resistor controls the bias of said amplifier, means included in said connections for producing an initial bias upon the input of said amplifier, an output circuit connected to said anode and said cathode, whereby there is developed therein an amplified detected signal which is substantially undistorted regardless of the strength of signal input, and an audio-frequency amplifier coupled to said output for further amplifying said signal.

2. In an automatic amplification control radio receiver, an arrangement for producing a substantiallyundistorted output which comprises a detector circuit including a variable mu vacuum tube having cathode, grid, and anode, a tuned input circuit connected to said grid, a grid-leak resistor connected betweensaid cathode and said tuned input circuit, a condenser in shunt with said grid-leak resistor, a radio-frequency amplifier circuit the output of which is coupled to said tuned input circuit, connections from the input of said amplifier circuit to terminals of said grid-leak resistor, whereby the voltage across said resistor controls the bias of said amplifier, means included in said connections for producing an initial biasupon the input of said amplifier, and

an output circuit connected to said anodeand said cathode, whereby there is developed in said output circuit an amplified detected signal which is substantially undistorted regardless of the strength of the signal input.

3. In an automatic amplification control radio receiver, an arrangement for producing a substantially undistorted output which comprises a detector circuit including a variable mu vacuum tube having cathode, grid, anode, and screen electrode, a tuned input circuit connected to said grid, a grid-leak resistor connected between said cathode and said tuned input circuit, a condenser in shunt with said grid-leak resistor, a radio-frequency amplifier circuit the output of which is coupled to said tuned input circuit and the input of which is adapted to have impressed thereon a radio frequency signal, connections from the input of said amplifier circuit to terminals oi the grid-leak resistor, whereby the voltage across said resistor controls the bias of said amplifier, a direct electrical connection between said screen electrode and said anode, and an output circuit connected to said anode and said cathode, whereby there is developed therein an amplified detected signal which is substantially undistorted regardless of the strength of the signal input. I i

4. An automatic amplification control radio receiver circuit, which comprises a detector circuit including a variable mu vacuum tube having input and output electrodes, a turned input circuit connected to one of said input electrodes, a grid-leak resistor connected between theother of said input electrodes and said tuned input' circuit, and a condenser in shunt with said gridleak resistor, a radio-frequency amplifier circuit having an input electrode upon which is impressed a radio frequency signal and the output of which is coupled to said tuned input circuit, connections from the input of said amplifier circuit to terminals of said grid-leak resistor, whereby the voltage across said resistor controls the bias oi the input electrode 01 said amplifier, and an output circuit connected to said output electrodes, whereby there is developed in said out put circuit an amplified detected signal which is substantially undistorted regardless of the strength or the signal input.

5. In an automatic amplification control radio receiver, an arrangement for producing a substantially undistorted output which comprises a detector circuit including a variable mu vacuum tube, having, input and output electrodes, an input "circuit including a grid-leak resistor connected between said input electrodes, a condenser in shunt with said grid-leak resistor, a radio-Irequency amplifier tube the output or which is coupled to said input circuit, and means for controlling the amplification of said amplifieriby the voltage developed across at least a portion or said grid-leak resistor, whereby there is developed in the output of said detector a substantially undistorted detected signal regardless of the strength of the signal input.

6. In an automatic amplification control radio receiver, a detector circuit which includes a variable mu yacuum tube having a screen electrode and an anode, a direct connection between said screen electrode and anode,-a grid-leak resistor in the input of said tube adapted to have im-,

said resistor to a control element of said amplifier, whereby there is produced in the output of said tube a substantially undistorted detected signal regardless of the voltage in the input of said 7. In an automatic amplification control radio receiver, a radio-frequency amplifier having a control electrode, a detector circuit which includes a variable mu tube having a screen electrode and an anode, a direct connection between said screen electrode and anode, a grid-leak resistor in the input oi. said tube adapted to have impressed thereacross a voltage dependent upon the received signal intensity, means for impressing a voltage derived from the voltage across said resistor upon said control electrode, the firstmentioned voltage being insufilcient to cause serious plate rectification in said detector tube, and there is produced a substantially undistorted detected signal in the output said tube regardless of the voltage in the input thereof.,

8. In an automatic amplification control radio receiver, an arrangement for producing a subsantially undistorted output which comprises a detector circuit including a variable mu vacuum tube having cathode, grid, and anode, a tuned input circuit connected to said grid, a grid-leak resistor connected between said cathode and said tuned input circuit, a condenser in shunt with said grid-leak resistor, a radio-frequency amplifier circuit the output of which is coupled to said tuned input circuit, connections from the input of said amplifier circuit to terminals of said gridleak resistor, whereby the voltage across said resistor controls the bias of said amplifier, means included in said connections for producing an initial bias upon the input of said amplifier, and an output circuit having a low direct-current resistance and asubstantial high-frequency impedance connected to said anode and said cathode, whereby there is developed in said output circuit an amplified detected signal which issubstantially undistorted regardless of the strength of the signal input.

9. In an automatic amplification control radio receiver, an arrangement rorproducing a substantially undistorted output which comprises a detector circuit including a variable mu vacuum tube having cathode, grid, anode and screen electrode, a tuned input circuit connected to said grid, a grid-leak resistor connected between said cathode and saidtuned input circuit, a condenser in shunt with said grid-leak resistor, a radio-frequency amplifier circuit the output of which is coupled to said tuned input circuit, connections from the input of said amplifier circuit,to terminals oi the grid-leak resistor, whereby the voltage across said resistor controls the bias 0! said amplifier, a direct electrical connection between said screen electrode and said anode, and an output circuit having a low direct-current resistance and a substantial high-frequency impedance connected to said anode and said cathode, whereby there is developed in said output circuit an amplified detected signal which is substantially undistorted regardless of the strength of the signal input.

10. In an automatic amplification control radio receiver, an arrangement for producing a substantially undistorted output which comprises a detector circuit including a variable mu vacuum tube, having input and output electrodes, an input circuit including a grid-leak resistor connected between said input electrodes, a condenser in shunt with said grid-leak resistor, a radio-frequency amplifier tube the output of which is coupled to said input circuit, means for controlling the amplification of said amplifier by the voltage developed across said grid-leak resistor, and an output circuit having a low direct-current resistance and a substantial high-frequency reactance, whereby there is developed in the output a substantially undistorted detected signal regardless of the strength of the signal input. 11. In an automatic amplification control radio receiver; a radio-frequency amplifier having a control electrode, a detector circuit which includes a variable mu detector tube, a direct connection betweenjthe screen electrode andanode of said tube, a grid-leak resistor in the input of said tube and adapted to have impressed thereacross a rectified voltage insufficient to cause serious plate rectification in said detector tube, means for deriving from said resistor and applying to said control electrode a voltage which varies according to the voltage across said resistor, and an output circuit having a low directcurrent resistance and a substantial high-free quency reactance, whereby there is produced a. substantially undistorted detected signal in the output regardless of the voltage in the input thereof.

12. An automatic amplification control receiver which includes a radio-frequency amplifier, an amplifying vacuum tube having a grid and a plate and having its input coupled to the output of said amplifier, means for causing grid rectification of the signals impressed upon the input of saidtube by said amplifier, means for maintaining the value of plate rectification substantially less than the value of grid rectification as the input intensity of said signals increases, an

output circuit connected with said plate, having a low' direct-current resistance and a substantial high-frequency reactance, and means included in the input of said vacuum tube for controlling the radio-frequency amplification.

, 13. In a signaling system, a detector comprising a vacuum tube having input and output circuits, means for applying signals to said input circuit of such intensity that rectification ocours in both said circuits, said detector being of the variable-mu type whereby the rectification in said output circuit is maintained relatively small as compared with the rectification in said input circuit.

14. In a signaling system, a signal detecting arrangement comprising a vacuum tube having a cathode, anode and grid, means for impressing signals effectively between said grid and cathode, and a resistance shuntedby a condenser efiectively connected in circuit between said grid and i cathode, whereby rectification of said signal occurs in said grid-cathode circuit, said signals being of suificient magnitude to produce some rectification in the anode-cathode circuit, said grid being of the variable-mu construction whereby the anode-circuit rectification is maintained at a-small value as compared with the grid-cathode circuit rectification.

15. In a signaling system, an amplifier including an amplification control element, a device comprising two rectifying electrodes and a third ing an amplification control element, a grid-leak detector, an input circuit for said detector ineluding means for transferring signals from said amplifier to said detector, anioutput circuit for said detector, means for imparting to said-detector the characteristic of continuously varying amplification over the normal range of amplitude of said signals, means for deriving control energy from the input rectification action of said detector upon said signals, and means for supply- I ing the control energy to said amplification control element. a

1'7. In a carrier wave receiving system, an antplifier including an amplification control eleinent, a grid-leak detector comprising a variable mu thermionic device, means for deriving control energy from the rectifying action of said detector, and means for supplying said control energy to said amplification control element.

18. In a signaling system, a detector including input and output circuits and means for imparting thereto an amplification characteristic varying continuously, over an extended range of signal input intensities, within limits of at least two to one.

19. In. a signaling system, a detector including input and output circuits and a grid leak and condenser included in said input circuit, said detector including also means for imparting thereto an amplification characteristic varying continuously, over an extended range of signal intensities, within limits of at least two to one.

20. In a signaling system, a detector including input and output circuits and means for imparting thereto an amplification characteristic varying continuously, over an extended range of signal input intensities, within limits of at least two to one, and means for automatically main- I taining the intensity of the signals applied to said input within said range.

J OEN KELLY J OHNSON. 

